The operator of a vehicle can normally sense when one of the pneumatic tires on the vehicle becomes deflated because of a tilting of the vehicle itself. However it is not as easy to sense the occurrence of tire deflation in a trailer which is being towed along a roadway behind the vehicle being driven. This is particularly true where the tow trailer is of a size and dimension relative to the vehicle such that the tires of the tow trailer are not easily observed. Sensing a low tire on a trailer is also difficult if it is connected to the vehicle through a ball hitch which does not translate tilting motion of the trailer caused by tire deflation. Further, tow trailers are often pulled along a roadway at night, such that viewing the trailer is difficult. Further, constant attention to and observation of a trailer being towed can be distracting and particularly annoying to a driver with a utility trailer, a boat trailer, a recreational camper trailer, or another similar tow trailer.
In the past, various tire deflation warning devices and low tire indicators have been designed which attach to an axle adjacent a tire. Typically, such devices have an elongated rod, which is spaced slightly above the ground and is guided straight up and down for vertical movement upon contacting the road surface due to deflation of the tire. Vertical movement of the rod activates an electrical contact switch which is connected to a warning signal. Such devices, for example, are disclosed in U.S. Pat. No. 2,639,342 issued to Cope in 1953 and U.S. Pat. No. 5,032,822 issued to Sweet in 1991. It has been found that vertical actuation of the rod is not likely to occur without a bending action also, unless the deflation occurs while the vehicle is stopped. If deflation occurs while the vehicle is moving, the frictional forces at the point of contact between the vertically actuatable rod and the roadway surface will tend to bend the rod and thereby render the device inoperable. Rolling elements at the ground contacting end of the sensor rod, such as that disclosed in the Sweet '822 patent are designed to reduce this problem. Rolling elements of any effective size will cause the cost of the device to be substantially increased. Small diameter rolling contact, with a roadway of normal roughness, is not likely to avoid bending the actuation rod.
Another tire pressure indicator, as disclosed in U.S. Pat. No. 2,740,007, provides for a small amount of angular movement, as well as vertical movement, of the actuation rod. It is the small amount of angular movement which first causes an electrical contact, thereby indicating a deflated tire condition. However, the range of movement of such a device in the '007 patent is not sufficient to insure against damage where deflation occurs rapidly, as it does, for example, in a complete blowout situation.
Various other devices have been disclosed in which the sensor rod is constructed of a hollow coil spring with a flexible cable extending through the center of the coil spring. Examples of such devices are shown in U.S. Pat. Nos. 3,096,410 issued to Anderson in 1963 and U.S. Pat. No. 3,265,822 issued to Moulton in 1966. Both of these devices rigidly connect the upper end of a coil spring to the housing of the detection device. The coil spring has a distal end having a ball or a cap which will contact the roadway surface upon tire deflation. Between the contacting ball or cap and the rigidly connected upper end of the spring, a fixed length cable is connected. Thus, upon contacting the surface the spring is bent backward, thereby increasing its length due to the arc, so that relative movement of the fixed length flexible cable through the center of the coil spring activates a switch mechanism, thereby signalling the deflated tire condition. These devices require a relatively complex and expensive spring and internal cable mechanism plus connecting linkage and a switch mechanism which is relatively complicated, and subject to wear. Further, the entire mechanism is adversely affected by corrosion. For example, dirt and road grime can cause malfunction. Further, if used in wet weather conditions, or on a boat trailer which is periodically submerged in water, the apparatus may corrode and jam or the switch may fail or otherwise adversely affect reliability.
Other flat tire signal devices have been disclosed in U.S. Pat. No. 2,640,119 issued to Bradford, Jr. in 1953 and another U.S. Pat. No. 3,502,829 issued to Reynolds in 1970, which devices activate electrical switch contact upon rotation of a rigid sensor rod about a pivot axis located in the signal device housing. Such devices depend upon a straight tension spring acting through a lever arm to hold the sensor rod in a vertically downward position. Rotation of the vertical rod about the pivot axis works against the linear tension spring to actuate an electrical switch for a warning signal. Neither of these devices provide for a pivot shaft for activating a simplified electrical switch, the moving parts of which are completely sealed in a weather resistant and water-tight housing. Further, there is no disclosure of any mechanism for flexible compensation of a vertical component of force which may be imparted into the rotation mechanism upon tire deflation.